This evening, United Airlines experienced a computer outage interrupting the airline’s flight departures, airport processing and reservations systems. As a result, customers may experience difficulties completing transactions at united.com and United reservations. Our technology team is working to resolve the issue as soon as possible. We apologize for the disruption being caused to travelers at affected airports and we are seeking to resume operations as quickly as possible.

You can follow the United Airlines twitter account @united for getting the latest updates.

As of now, the latest update is " We're still trying to fix the issue. We will keep you posted."

Update
It seems the United Airlines solved the issue. Their latest tweet says "Our systems are up. We are in the process of resuming ops for UA. We again apologize for the disruption caused to our customers."

And, United Airlines has issued a waiver for travel plans impacted by the computer outage on June 17 & 18.
You can see the details United Airlines website.